1. Field of the Invention
This invention relates to a device for detecting deterioration of a catalyst (catalytic convertor) of an internal combustion engine, and more particularly to a device for detecting deterioration of a catalyst arranged in the exhaust system of the engine by the use of an output from an exhaust component concentration sensor downstream of the catalyst or the like.
2. Prior Art
Conventional catalyst deterioration-detecting devices for internal combustion engines include a device which is disclosed in Japanese Provisional Patent Publication (Kokai) No. 2-30915. According to this prior art, a three-way catalyst is arranged in an exhaust system of an internal combustion engine, and two oxygen concentration sensors (hereinafter referred to as "the upstream O2 sensor" and "the downstream O2 sensor" as exhaust component concentration sensors are arranged upstream and downstream of the three-way catalyst, respectively. These O2 sensors have such an output characteristic that their output levels thereof are inverted as the air-fuel ratio of a mixture supplied to the engine changes across a stoichiometric air-fuel ratio. The device according to the prior art detects deterioration of the three-way catalyst, based on a time period elapsed from the time the air-fuel ratio of the mixture changes to a rich value with respect to the stoichiometric air-fuel ratio to the time the output from the downstream O2 sensor is correspondingly inverted from a lean side to a rich side with respect to a reference output value, or a time period elapsed from the time the air-fuel ratio changes to a lean value with respect to the stoichiometric air-fuel ratio to the time the sensor output is correspondingly inverted from a rich side to a lean side with respect to the reference output value.
More specifically, as shown in FIG. 1, for example, a time period is measured, which elapses from a time point t1 the output V1 from the upstream O2 sensor changes to the rich side to a time point t2 the output V2 from the downstream O2 sensor is correspondingly inverted from the lean side to the rich side. Based upon the thus measured time period, it is determined whether the three-way catalyst is deteriorated.
This deterioration-detecting technique is based upon the following ground:
A three-way catalyst in general has a so-called oxygen storage effect that it takes in oxygen when the air-fuel ratio lies on the lean side, whereas it takes in CO and HC in exhaust gases from the engine when the air-fuel ratio lies on the lean side. As the oxygen storage effect is greater, a time difference between a time point of a change in the air-fuel ratio of exhaust gases in the exhaust system upstream of the catalyst and a time point of a corresponding change in the air-fuel ratio of exhaust gases downstream of the catalyst becomes larger. The above-mentioned measured time period represents this time difference. The oxygen storage effect becomes smaller as the degree of deterioration of the catalyst is higher, so that the measured time period becomes shorter. Therefore, when the measured time period is shorter than a predetermined time period, it can be judged that the oxygen storage effect is degraded, i.e. the catalyst is deteriorated.
According to the prior art, the detection of the catalyst deterioration is carried out when the engine is in a condition where the supply of fuel is increased or when the engine is under fuel cut so that a change in the output characteristic of the upstream O2 sensor does not affect the output from the downstream O2 sensor, thereby making it possible to detect deterioration of the catalyst with high accuracy.
However, the prior art device has the disadvantage that the detection of the catalyst deterioration has to be carried out only when the engine is in a particular operating condition other than an air-fuel ratio feedback control condition, such as a condition where the fuel supply is increased, and a condition where fuel cut is carried out, and further, when the catalyst deterioration detection is carried out, the air-fuel ratio of the mixture supplied to the engine has to be set to a rich value or a lean value with respect to the stoichiometric air-fuel ratio. Therefore, in carrying out the catalyst deterioration detection, it is necessary to wait until the engine enters such a particular operating condition or temporarily interrupt the air-fuel ratio feedback control. Thus, the catalyst deterioration detection cannot be carried out concurrently with the air-fuel ratio feedback control. Moreover, the exhaust emission characteristics may be degraded during the catalyst deterioration detection due to the temporary interruption of the air-fuel ratio.
Besides, according to the prior art, it takes a considerable time to complete the catalyst deterioration detection, since the time period before occurrence of the inversion of the output from the downstream O2 sensor has to be measured.